1. Field of the Invention
The present invention concerns an apparatus for producing metal powder by gas-atomizing method. The apparatus is particularly useful for producing powder of special steels and super alloys, and it is possible to produce very clean metal powder with a preferable embodiment of this apparatus.
2. State of the Art
Powder metallurgy has been often used for production of tools from a high-speed steel or sintered hard alloys of a high carbon content, or production of parts of a jet-engine or a gas-turbine with a Ni-based or a Co-based super alloy. By recent progress in HIP technology and spread of the equipments of large capacities, it is getting easier to produce the parts of desired shapes and performance starting from the metal powders.
As a method of producing metal powder of low impurity contents, it has been known to atomize the molten metal with a jetting gas. The gas-atomizing method is carried out by using a molten metal atomizing appratus comprising a molten metal holding vessel equipped with a molten metal discharging runner at the bottom thereof and a spraying chamber equipped wiht gas-jetting nozzles therein. There was proposed to use a discharging runner equipped with a sliding gate at the lower end of the runner connected to the bottom of the vessel and an induction heating coil around the runner.
In the discharging runner mentioned above, when the metal in the runner is heated with the magnetic flux given by the induction coil surrounding the runner, it is often observed that the flux extends to the sliding gate to cause heating of the gate. Therefore, it is necessay to choose a heat-resistant material or a non-electroconductive material as the material of the gate. Also, consumption of the flux or loss of electric power is inevittable at the gate. If the distance is so large that the flux from the coil may not extend to the gate, then it will be possible that the molten metal solidifies in the runner.
With respect to the quality of the product metal powder, while practice of HIP at a higher temperature under a higher pressure gives products having bulk densities substantially the same as those of the ingot products, demand for better material metal powder is getting severer. In order to fully enjoy the merits of using powder metal, i.e., fine crystal grains, and fine precipitation and uniform distribution of the strengthening material such as carbides, it is necessary that the metal powder contains a very small amount of surface oxides and free from external impruitites such as pieces of refractories or slags.
The powder metal products produced by conventional gas-atomizing technologies are not satisfactory.